Method of anodically polishing aluminum



April 24, 1951 c. L. FAUST METHOD OF ANODICALLY POLISHING ALUMINUM Filed Nov. 14, 1947 3 Sheets-Sheet 1 2.533 urzuuum O m m m 1 s m 7W 1 SPECIFIC GRAHTY 0.8 L0 L2 95ALUMINUM 06 o wwmwmww 'EVEZYTEI" 5o Camus A. film" 4.0 DISSOLVED ALuMmumx April 24, 1951 c. FAUST METHOD OF ANODICALLY POLISHING ALUMINUM 3 Sheets-Sheet 2 Filed Nov. 14, 1947 55.223 wzrimmmo 2 $29558 5603;

O O O 0 O m m n w a @DP mun-2UP Q o O 0 0 0 8 2.0 2.5 3.0 -PER CENT ALUMINUM IN SOLUTION .ELQ. 5

7! ALUMINUM 0 o o A 3 5 FM m/w 7m R 5 D 10 0. 2 W 98m. w 4 5 o mmwm m o Fmdw mazfl rzwzwa Emma.

DISSOLVED- ALUMINUM Z April 24, 1951 C. L. FAUST METHOD OF ANODICALLY POLISHING ALUMINUM Filed Nov. 14, 1947 CURRENT DENSITY,

AMPS. 50. F1:

U L 2 40 r; 2 Ill 9 30 t 0 20 Q Q 22 0 I00 200 300 400 500 600 100 e00 900 I000- CURRENT DENSITY AMPERES PER SQUARE FEET U :2 I70 5 I '2 m I60 \ll I50 ll] I40 %ALUM|NUM F1 9 s00 400 I as 200 W loo JFK E2775? Cl/AAZAY L fill/5r 0 L0 2.0 3.0

5 Sheets-Sheet 5 DISS OLVED ALUMINUM X E/Z/vzs.

Patented Apr. 24, 1951 METHOD OF ANODICALLY POLISHING ALUMINUM Charles L. Faust, Columbus, Ohio, assignor, by mesneassignments, to Bat-telle Development Corporation, Columbus, Ohio, a corporation of Delaware Application November 14, 1947, Serial No. 786,092

3 Claims.

This invention relates to an electrolyte comprising anaqueous solution of sulfuric, phosphoric and 'chromic acids and tormethods: of anodically polishing aluminum and "aluminum alloys in said electrolyte.

In the anodic' electropolishing :of aluminum on a commercial scale, it is desirable to operate at a. relatively lowvoltage, for instance, from 7 to-15 volts; to operate at a relatively lowcurrent density, for instance, from 60 to 200 amperes per square foot; and'to operate with the same bath for a maximumlength of time or, at least, with-only such adjustments in the composition of said bath as can be carried out with: out interruption of polishing.

.'In my copending application, "Serial No. 569,641 filedDecember 23, 1944 (nowabandoned) and entitled Electrolyte for andrrMethod 'of Anodically Polishing Metals," of which'the :-pres ent application is a continuation-impart, I have disclosed electrolytes. adaptedfor polishing aluminum comprising an aqueous. solution: of sulfuric, phosphoricand chromic acids. However, although operativathe' compositions of I this ccopending application will not always electropolish aluminum at the commercially .mosti:desirable voltages or current densities or for such length of time (without reconditioning) as is most desirable for commercial purposes.

' It is therefore an important object of the presentinventionto provide electrolytes-"forand methods of anodically electropolishing aluminum operative at low voltages, low current densities and for long periods of time.

; Other and further objects'and features ofxthe present invention 'will' :become. apparent. from the following description, drawings; and appended claims.

The electrolytes according to .the'=.present .invention comprise essentially from 4 to 45% (.by weight) ofsulfuric acid (100% H2SO4), from .40 to 80% oi-phosphoric acid -(100% HsPOl), from 0.2% or preferably 0.4% .to saturation of chromic acid (100% G103) ,and from to 6% of dissolved. metals (aluminum,v or aluminum and trivalent chromium), the balance being essentially Water. The total acid concentration ranges from 50 to 95%. Such baths areoperative to electropolish aluminum at a voltage of '15 volts or less and at acurrentden'sity ranging from 25 to 950 amperes per square foot, the current densityvarying according to the viscosities of" the baths, as will be disclosed in greater de .tailhereinbelow. Preferably, the baths areoperat'ed at a current density "of from60 to "1'50 2 or 200 amperes per square foot, a temperature of from 160 to 200 F., a -viscosity of from 9t0 l3 centipoises (at the operating temperature of, say, 180 F.) and a voltage of from '7 to 15 volts. Suitable agitation of the bath is effected, the exact amount and type'depending on'the shape and size of the articles being polished, the number of articles being polished, the spacing ofthe articles and of 'the cathodes, the size and shape of the tank, and like factors.

A specific bath according to the present invention comprises 4.7% sulfuric acid, 75% phos phoric acid,- and 6.5% chromic acid, the balance being essentially water together with not more than 4.5% of dissolved aluminum-or of dissolved aluminum and trivalent chromium. This bath is preferably operated at a current density of from 60 to 150 amperes per squareio'ot, a temperatureof from l75'to 185 F. (with some variation depending on dissolved aluminum, or aluminum and trivalent chromium content, as described hereinbelow), a viscosity .of from 10 to about 11.5 centipoises, and a voltage of from 7 to 12 volts.

The practical significance of the'features disclosed hereinabove will be apparent from the following discussion.

Figure 1 is a graph. showing the permissible maximum concentrations of dissolved aluminum for electropolishing at not more than l5'volts in baths'having a relatively constant sulfuric acid plus phosphoric acid concentration, .and having the sulfuric acid concentrations shown in the abscissae.

Figure Zshows the relationship between jspecific gravity and viscosity of awpolishing bath containing 4.7%, sulfuric acid, phosphoric acid, and. 6.5% chromic acid with respectto the dissolved aluminum content.

Figure 3 is a chart showingthe temperatures required for the production of a mirrorfinish-on aluminum at a current density of '100 amp/sq. it. and a voltage of 12 to 15 volts with respect to variations in the dissolved aluminumcontent, using the same bath as in Figure 2.

Figure i'is a graph illustrating how the temperature must be increased and the specific grav ity' lowered in a bath containing 41.7% sulfuric acid, 75% phosphoric acid, and 6.5% chromic acid to maintain a. constant viscosity despite increase in the dissolved aluminum content.

Figure 5 is a graph. illustrating the-increase in voltage at various temperatures when using a bath containing 5% sulfuric acid, phosphorlc acid, and water containing amounts of dissolved aluminum.

Figure 6 shows the operating characteristics of a bath containing 15% sulfuric acid and 63% phosphoric acid with varying amounts of dis solved aluminum at an operating temperature of 200 F.

Figure 7 is a graph showing the viscosity in centipoises required for electropolishing at a voltage of 12.5 volts at various current densities using a bath containing 4.7% sulfuric acid, 75% phosphoric acid, and 6.5% chromic acid.

Figure 8 is a graph showing how the oath used in Figure must be operated at higher temperatures as the dissolved aluminum content is increased.

Figure 9 shows the operating characteristics of a bath comprising 45% sulfuric acid and 45% phosphoric acid and containing varying amounts of dissolved aluminum.

Figs. 3, 6 and 9 show certain combinations of current density, voltage and temperature operative for the production of a mirror polish on aluminum in baths containing various amounts of dissolved aluminum. The three baths" of Figs. 3, 6 and 9 are aqueous solutions of phosphoric and sulfuric acids containing various amounts of dissolved aluminum but devoid of chromic acid. The latter acid has been omitted varying from these baths for the reason that the chromic acid, except by providing a source of trivalent chromium, does not affect the electropolishing operation one way or another during the time the electropolishing current is kept flowing. The function of the chromic acid is that of pro tecting the mirror finish after the now of electropolishing current has been terminated, when the aluminum is subject to etching attack by a bath devoid of chromic acid.

In Fig. 3 are shown the temperatures required for the production of a mirror finish on aluminum when it is desired to electropolish at 100 amp/sq, ft. and to keep the voltage in the range of 12 volts to. 15 volts while the dissolved aluminum content varies. The figures within the circles show the voltage required to establish a current density of 100 amps/sq. ft. at the indicated dissolved aluminum content and at the temperature designated.

As shown by Fig. 3, aluminum can be electropolished to a mirror finish in a bath containing 5% sulfuric acid, and 80% phosphoric acid at a dissolved aluminum content ranging from 0 to 4.5% without any need for raising the cur rent density above 100 amperes per square foot or for raising the voltage above 15 volts. Further, it is not necessary to raise the temperature to a value of 200 F. until a dissolved aluminum. content of 3.0% has been reached.

Fig. 6 shows the operating characteristics of a bath containing 15% sulfuric acid and 63% phosphoric acid, the balance being essentially water together with various amounts of dis solved aluminum. In the case of this bath, all operating data relate to a temperature of 200 F., since electropolishing cannot be carried out below this temperature at relatively low current densities or voltages. Further, the data relate to contents of dissolved aluminum not greater than 1.5%, for the reason that at higher contents of dissolved aluminum electropolishing cannot be carried out at relatively low current densities or voltages. Each circle in the chart of Fig. 6 represents a current density operative at the indicated content of dissolved aluminum,

and the operating voltage for that particular current density has been inserted within each circle. Unlike Fig. 3, Fig. 6 shows not only the minimum current density operative at a particular content of dissolved aluminum, but various other and higher operative current den sities, together with the voltages required to maintain all these operative current densities. Fig. 9 relates to the operating characteristics of a bath comprising 45% sulfuric acid and 45% phosphoric acid, the balance being essentially water together with various amounts of dissolved aluminum. All the data of Fig. 9 refer to an operating temperature of 200 F. and a dissolved aluminum content of not more than 1.5%, for the reason that electropolishing cannot be effected with this bath at relatively low current densities or Voltages at temperatures of less than 200 F. or at contents of dissolved aluminum in excess of 1.5%. Otherwise, the chart of Fig. 9 makes the same showing as the chart of Fig. 6. Thus, as shown by the data in the graphs of Figs. 3, 6 and 9, a low sulfuric acid content (as compared to the phosphoric acid content) makes possible electropolishing at higher dissolved aluminum contents; at lower voltages; at lower current densities; and, at dissolved aluminum contents of 2.0 or less, at lower temperatures.

Reference is also made to Fig. 1 showing the permissible maximum concentration of dissolved aluminum for electropolishing at not more than 15 volts in baths containing sulfuric and phos phoric acids in various ratios but having approximately the same total sulfuric-phosphoric acid 1 concentration. As shown, quite high concentrations of dissolved aluminum permit electropolishing at not more than 15 volts in baths containing less than 10% sulfuric acid, while no electropolishing at 15 volts is possible in baths containing 65% 01' more sulfuric acid.

The above noted adverse eiiect of increasing the sulfuric acid content (at the expense of the phosphoric acid content) obtains normally at relatively high total acid concentrations (at about total acid concentration or higher). At lower total acid concentrations, a higher proportion of sulfuric acid will not interfere with electropolishing at from 10 to 15 volts. For instance, a bath containing 32.5% sulfuric acid, 42.6% phosphoric acid and 1.6% chromic acid, the balance being essentially water, will satisfactorily polish aluminum at 10 to 15 volts.

While, at higher total acid concentrations, best results are obtained with relatively low concentrations of sulfuric acid, the presence of these relatively small amounts of sulfuric acid is nevertheless necessary at all total acid concentrations for best results.

The operating characteristics of baths devoid of sulfuric acid are indicated hereinbelow. Baths containing from 50 to 85% phosphoric acid and, optionally, chromic acid, but no sulfuric acid, will electropolish aluminum but with some pitting of the aluminum or at undesirably high voltages, current densities or temperatures or at various undesirably narrow ranges of voltage, current density or temperature. For instance, for electropolishing in baths containing, respectively, 50% and 70% phosphoric acid, at amperes per square foot, there are required, respectively, voltages of 21 and 35 volts. In the case of a bath containing 75% phosphoric acid and 1.2% dissolved aluminum, the balance being essentially Water, aluminum can be electropolished at current densities of from 100 to 250 acid-Lea es some duster, butiiull brilliance is; not -29 usin fmmslfi tQAQ-Vottnd Atptlie De l ureav a adiiierer-it L-currientsdensitr .ra n e required; ,In thencase 'of baths, containing .50"

thsiibalance being; essentially water, electron p usnmaisefiectedat 189531? a purrentdensity, Y 110,550, amperes perisquaneiootgandn "frame- 10- togztevolts. Best electro The cperatina characteristicsbf baths contain-- A d], e ror 'nstance;

ldtO .bathnqiltalhing,

v cide entehtrtc15%analses pcssi le abriiliant. mirror: likeefinish o enefiit to, :be desired f om rthespre encefoil-r rench t sts i-ma bath containing;0;%iphosappori acid nd {1%, chrcrnicr-acid ;the,-ebalan er, ein rp een a lyraivater. This-zbathiisl the equivalelaticisihezg reterredbath-,ccntainingeessentially :1 4.1% ulfu chromi acid apartdronnthe. 1Llf,-11l3i6-,.&Gida;: In th d addjlilgfilflfllljifiacid to asbatheccne, ,tainingiefl ph snhcriaacid andfl chrcmicacidc wiiliwieldrthe preferredbathe In such as-bath,-

containine guai hospherici acid and ,7 chromic -35,

acid ;;elcctropplishingigtakes place at a current density of from 100 to 250 amperes per square foot, a voltagantzifrormfiltdzfitvoltsiand a tempera ureictlfilril Howev rhthe aluminum, although lustrous, ispltted. At 30.;t; 50,.,amperes per square foot, the luster is uneven and pitting is alsov evident? Attempts to electropolish at lower temperatures in the bath: containing 80%" pimsphcric; acid .andlhrhrcmic; acid either-dead. tcr-an dizineaor. require excessivewdtas se;

"Wit r spect to, thecnromicacid of the-acid ontention-rm bath pithe. present invention; lit: should be noted ,that, ,aepointed southereinabove, the chromic acid isfadded to prevent chemical phosphoric acid and 4 2%.Lidissoived aluminum; 5

however, s is: efiected-W at, a current isr-ifurtherr demonstratedl by; elec $25,

I cathodechamberlwhere itdoesnot acttd shorten theliie oftne. electrcpolishingsbathr.. Thusathereis,nclsignificantricrmaticn oiltrivalentchromium.n outside of .thenporous cathode-.cupsand ,theselece. tropo'li-shingl liie, is, .raccordingly, ,not. shortened. by; such formation; Theuuselofi porous cups tfors. preventing, or. greatly. retarding; such. cathodic; reduction is,,well*known.inthefieldpt electrolysis, andrdetails :ior, effecting the. =resu1t \desiredrmays beyaried inaccordance with; sucli.knowledgeel 'Il;1e, general operating conditions fonthe baths-s: oi. the present. invention have, beenn disclosed m hereinabove. More specific details; are given'e hereinhelow..- 1

Ref-erenceris made, toathe. graph, ofhFigure r, Thi grapnlshows' theaviscosity, (in centipoisesas, required for electropolishinge iati constant voltage; e

. (1.2.5.,vclts) at ,various current densitiesfor a bath initiall ,.containin nj sulfuriciacid, ;75% phos-s phoric'. acid, landfirfi chromiciacid, the: balance being ,essentially water .As, shown the-:viscosi must ,be lowered as. higherf current densities are 11 applied if, ithervoltage is (to be lmaintained q atmaa. constant value. Such lowering pf,,viscositinmaiy be .eilectedby dilutmnwith ,water, by-bperatian at. a I higher tern.perature,-. on lay-both, methods asls will, be, discussed inter eaten detail ,hereinbelowm.

In, the precedingnparagraph.it has been shown that [the initial ,rviscosity. ,must be regulated if; i

acid, 5;%;.phosphoric acid and-l6.5 1 730, higher currentdensities are tolce applied without:

the. use of an excessively highyoltage, Howeven, as the dissolved aluminum content, (and alsoithei. trivalent chromium content) of. a .bathlincreasess when the bath is, used for,electropolishing aluw aminum, this increased dissolved metal content causes an-increase both in the specific gravity and i inthe, viscosityof the bath. Referenceis made to'Figure 2 "as showing the, specific gravity andQ viscosity atvarious dissolved aluminum contents;

w At'these increased specific gravities and. viscos ities, higher volta es must be applied to maintain l the current density, and the operating temperature must also be raised. Figure 5 shows graphit cal-1y" the-increase iri'volta ge at various tempera-e,

A tures' a bath containing 5%"sulfuricacid,8U%ff phosphoric acid jand "water, "at various contentsf of dissolved. aluminum." Reference. is also. made' to'Fia. 8 as showing. howthe operating tempera,-

turefmustbe raised as the dissolved-aluminum;

attaclaof theal-umi nurn 'aft'er termin ation of=-e'lec- 63 o tent is increased.

tropolishinc; and whi le the-polished-aluminum is still wetted with the polishing bath. At least 0.4% chromic acid should be incorporated with the bath for best results. Chromic acid may be present in any amount from 0.2% to saturation. lit

The presence of chromic acid does not immediately afiect the electropolishing. However, any trivalent chromium formed as a result of the reduction of chromic acid has the same effect as dissolved aluminum. ence of trivalent chromium will lower the permissible concentration of dissolved aluminum so that the life of the bath is correspondingly shortened.

The formation of trivalent chromium is caused 65 by cathodic reduction of the chromic acid. This action can be prevented by using porous ceramic media around the cathode. These media form a closed cup in which 85% phosphoric acid is used as a catholyte to carry the current to the cathodes placed therein. Theporous medium around the cathodes decreases the amount of chromic acid entering the reducing environment. Whatever chromic acid does enter the catholyte is reduced to trivalent chromium which remains in the In other words, the pres- 00 4 "shows how the temperature must "'be" increased and the specific gravity lowered (by the addition of water), in a bath containing 4.5% sulfuric acid, 75% phosphoric acid and 6.5% chromic acid, the balance being essentially water, if the viscosity is to be'maintained constant in spite of an increased dissolved aluminum content due to continued operation.

In general, there is an upper limit of aluminum concentration, which is reached in the use of the baths herein disclosed and which is dependent upon the sulfuric acid/phosphoric acid ratio and the total acid concentration. c

It is thus apparent that, while the baths of the present invention are operative at current densities of from 25 to 950 amperes per square foot, the desired initial low voltage conditions are not obtained except by regulation of the initial viscosity of the bath. Further, if the desired low voltage is to be maintained throughout the I "life of the bath, the initial viscosity should be maintained at its original value, or within the limits indicated, as by addition of water to the bath and/or increasing the operating temperature, or both.

The methods of this application are operative to electropolish aluminum and aluminum alloys containing at least 90% aluminum but not over 1% silicon. Examples of such-metals are commercially pure aluminum (99.4 to 99.0% aluminum, 1.0 to 0.6% iron, and small amounts of silicon and copper); commercially pure aluminum containing 1.2% manganese; an alloy of commercially pure aluminum containing 4% copper, 0.5% manganese and 0.5% magnesium; an aluminum alloy containing 1.9% copper, 0.6% magnesium, 1.0% silicon and 96.5% aluminum; an alloy containing 2.5% magnesium and 0.25% chromium, the balance being essentially aluminum; and an alloy containin 4.4% copper, 0.8% silicon, 0.8% manganese, 0.4% magnesium and the balance essentially aluminum.

Many details of composition and procedure may be varied within a wide range without dee parting from the principles of this invention and it is therefore not my purpose to limit the patent granted on this invention otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. A method of electropolishing aluminum and aluminum alloys containing at least 90% aluminum and not over 1% of silicon, which comprises making the aluminum the anode in a composition initially comprising essentially from 4 to 45% by weight of sulfuric acid, from to 80% by weight of phosphoric acid, and from 0.2% by Weight to saturation of chromic acid, maintaining a total amount of dissolved aluminum and trivalent chromium during continued operation not in excess of 6%, the balance being essentially water, and the total acid concentration being from 50 to 95% by weight of said composition, and passing a current through said composition ata temperature of from 160 to 200 F., an anodic current density of from 25 to 950 amperes per square foot and a voltage not in excess of 15 volts.

2. A method of electropolishin aluminum and aluminum alloys containing at least 90% aluminum and not over 1% of silicon, which comprises making said aluminum the anode in a composition initially comprising essentially from 4 to by weight of sulfuric acid, from 40 to 80% by weight of phosphoric acid, and from 0.2% by weight to saturation of chromic acid, main- 8 taining an amount of dissolved aluminum such that the total amount of dissolved aluminum and trivalent chromium during continued operation is not in excess of 6% by weight, the balance being essentially water and the total acid con centration being from to- 95% by weight, and passing a current through said electrolyte at an anodic current density of from to 150 amperes per square foot, at temperature of from 160 to 190 F., and a voltage of from 7 to 15 volts, and maintaining said composition during said electropolishin at a viscosity of from 9 to 13 centipoises as measured at operating temperature;

3. A'method of electropolishing aluminumand aluminum alloys containing at least aluminum and not over 1% of silicon, which comprises making said aluminum the anode in a composition initially comprising essentially from 4 to 45% by weight of sulfuric acid, from 40 to 80% by Weight of phosphoric acid, and from 0.2 by weight to saturation of chromic acid, maintairiing an amount of dissolved aluminum such that the total amount of dissolved aluminum and trivalent chromium during continued operation is not in excess of 6% by weight, the balance being essentially water and the total acid concentration being from 50 to by weight, and passing a current through said electrolyte at an anodic current density of from 60 to amperes per square foot, at a temperature of from to F., and a voltage of from 7 to 15 volts, and adding water as necessary to said composition to maintain said composition during said electropolishing at a viscosity of from 9 to 13 centipcises as measured at operating temperature.

" CHARLES L. FAUST.

' REFERENCES CITED The following references are of record in thev ciety, June 1946, pp. 50, 51, 67. 

1. A METHOD OF ELECTROPOLISHING ALUMINUM AND ALUMINUM ALLOYS CONTAINING AT LEAST 90% ALUMINUM AND NOT OVER 1% OF SILICON, WHICH COMPRISES MAKING THE ALUMINUM THE ANODE IN A COMPOSITION INITIALLY COMPRISING ESSENTIALLY FROM 4 TO 45% BY WEIGHT OF SULFURIC ACID, FROM 40 TO 80% BY WEIGHT OF PHOSPHORIC ACID, AND FROM 0.2% BY WEIGHT TO SATURATION OF CHROMIC ACID, MAINTAINING A TOTAL AMOUNT OF DISSOLVED ALUMINUM AND TRIVALENT CHROMIUM DURING CONTINUED OPERATION NOT IN EXCESS OF 6%, THE BALANCE BEING ESSENTIALLY WATER, AND THE TOTAL ACID CONCENTRATION BEING FROM 50 TO 95% BY WEIGHT OF SAID COMPOSITION, AND PASSING A CURRENT THROUGH SAID COMPOSITION AT A TEMPERATURE OF FROM 160* TO 200* F., IN ANODIC CURRENT DENSITY OF FROM 25 TO 950 AMPERES PER SQUARE FOOT AND A VOLTAGE NOT IN EXCESS OF 15 VOLTS. 